The Royals announced the extent of the injury after Duffy underwent an MRI examination on Monday in Kansas City. The injury was diagnosed by head team physician Dr. Vincent Key of the University of Kansas Medical Center. Duffy will get a second opinion from Dr. Lewis Yocum of Los Angeles at a time to be determined.
"The bad news is -- yeah, it stinks -- he'll probably be out a year," manager Ned Yost said. "The good news is when he comes back, the way they've been doing these surgeries, he's going to be every bit as strong or stronger, and ready to resume his career."
Duffy was placed on the 15-day disabled list and right-hander Nate Adcock was brought back from Triple-A Omaha to take his roster spot. Adcock was optioned to Omaha on Saturday when reliever Greg Holland was activated off the disabled list. Adcock had no record and a 2.89 ERA in three games with Kansas City.
Duffy was removed with tightness in his pitching elbow after facing just three batters in Sunday's 9-1 win over the White Sox in Chicago. A similar problem caused him to skip a start on April 27, but Sunday's was the third start he'd made since then.
The missed start was caused by a flexor strain in his elbow, but afterward, he regained his range of motion and velocity on his pitches. On May 3, he pitched 5 1/3 innings of a 4-3 victory over the Yankees, and last Tuesday, he went 4 1/3 innings of a 6-4 win over the Red Sox. In the latter game, he was relieved after making 102 pitches and had a no-decision.
"There was no indication between his last start and this start to show that there was anything wrong with it," head athletic trainer Nick Kenney said. "No increase in stiffness, no increase in treatment, no backoff of sides [practice sessions]. It was normal, nose-to-the-grindstone fifth-day stuff."
But Duffy felt a tugging in his elbow on the second pitch he threw in Sunday's game against the White Sox. He faced three batters and wanted to continue, but Yost and Kenney decided to take him out of the game.
Kenney said that Duffy has had a tear in his ulnar collateral ligament since 2010, but, like many pitchers, was able to adapt and continue pitching until Sunday.
If reconstructive elbow surgery is decided upon for Duffy, he'll be the second Royals pitcher to have the procedure this year. Closer Joakim Soria underwent Tommy John surgery performed by Dr. Yocum on April 3.
In six starts this season, Duffy has a 2-2 record and a 3.90 ERA with 28 strikeouts and 18 walks in 27 2/3 innings.
Long one of the Royals' prime pitching prospects, Duffy emerged from the Minors last season in mid-May and made 20 starts, compiling a 4-8 record and 5.64 ERA before being shut down in early September because of a limit on innings pitched. Counting his eight starts with Triple-A Omaha, Duffy worked 147 1/3 innings last year.
In 2010, he left the Royals' Minor League camp during Spring Training to take stock of his future. He returned to baseball that June and eventually pitched for four different Minor League clubs. He was back in the Royals' Major League camp in 2011 and began the season with Omaha.
The Kansas City pitching staff has been hit hard by injuries this season, including a major blow in Spring Training when Soria was diagnosed with a torn ulnar collateral ligament in his right elbow. It resulted in the second Tommy John surgery for Soria.
Reliever Blake Wood also was a spring casualty with a right ulnar nerve irritation. Felipe Paulino went on the disabled list during training camp with a right forearm strain, but he has since returned to make two starts.
Holland was out from April 21 until last Saturday with a rib injury. Starter Jonathan Sanchez went on the DL last Wednesday with biceps tendinitis in his pitching arm.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.